Corrugated paperboard is widely used as a material for fabricating containers and for other packaging applications. Corrugated paperboard is strong, lightweight, relatively inexpensive, and may be recycled. Conventional corrugated paperboard is constructed of two opposing liners and an intervening fluted sheet secured together using an adhesive. The adhesive is typically a starch-based adhesive applied as a liquid. Accordingly, heat is transferred to the adhesive to dry or set the adhesive during manufacturing of the corrugated paperboard.
Referring to FIG. 1, a conventional so-called "double facer" 20 for setting adhesive is schematically illustrated. The double facer joins a "single faced" corrugated paperboard sheet, including a single liner and the fluted paper, together with a second liner and heats the sheet to dry or set the adhesive. This heating is typically achieved by passing the corrugated paperboard over a series of steam heating chests from left to right as shown in the illustrated double facer 20. The heating chests are typically grouped together in heating sections 21a-21d. More particularly, the corrugated paperboard is advanced over the series of steam heated chests by an endless conveyor belt 23 and its associated drive 24 to engage the sheet and advance the sheet in contact with the underlying heating chests. A lower traction belt 26 assists in advancing the sheet through the double facer.
Referring now to the cross-sectional schematic views of FIGS. 2 and 3, in a conventional double facer, a series of transversely extending rolls 25 are carried by side rails 24 (FIG. 1) and provide a downward or backing pressure on the back side of the conveyor belt 23. Thus, the rolls 25 of a conventional double facer are intended to provide a backing force to facilitate contact between the advancing corrugated sheet 27 and the underlying heating surfaces 28 of the heating chests 22. See also, for example, U.S. Pat. No. 4,316,755 to Flaum et al. and U.S. Pat. No. 3,981,758 to Thayer et al.
As shown in FIG. 2, when the apparatus is first started, the upper heating surface 28 of the heating chest 22 is substantially planar. Accordingly, the sheet 27 is evenly pressed across the heating surface by the liner backing rolls 25, and uniform heating and setting of the adhesive may be obtained. Unfortunately, as shown in FIG. 3, the heating chests 22 have a tendency to bow inward at their centers as a result of temperature differences in the walls of the heating chest. The bowing is typically more severe in an upstream heating chest 22 since the cooler corrugated paperboard produces a larger temperature differential in the upstream heating chest.
The thermally induced bow may cause a loss of contact at the center of the heating surface 28 as illustrated by the gap 30. Accordingly, heat is not properly transferred into the adjacent portion of the corrugated paperboard sheet, resulting in scrap or an inferior product, such as a sheet having blisters. Moreover, the edge portions of the sheet 27 must support the full weight of the rolls 25 as indicated by the downward directed arrows which, in turn, typically results in crushed edge portions of the sheet. The crushed edge portions also produce an inferior product or scrap.
Another drawback of backing rolls is that a relatively large number of heating chests 22 must be used to ensure that all portions of the sheet, particularly the center portion, obtain sufficient heat to set the adhesive. The additional heating chests 22 have ambient energy losses; hence, the overall energy efficiency of the process is reduced. Moreover, the additional heating chests fail to address the problem of edge crush of the corrugated paperboard sheet also caused by bowing of the heating chests.
One proposed attempt for providing uniform backing pressure to the advancing corrugated paperboard sheet is disclosed in U.S. Pat. No. 3,319,353 to Matsunami et al. The patent discloses a plurality of air chambers each having an open bottom through which compressed air from a blower is directed onto the back side of the conveyor belt. An elastic material seals the junction between the belt and the air chambers to prevent leakage. In one embodiment, the conveyor belt is porous so that the compressed air may be directed onto the corrugated sheet to further facilitate drying.
Another approach to compensate for bowing of the heating chests, is a roll system wherein each roll has a padded covering or an enlarged medial portion to attempt to conform to the bowed heating chest surface. These approaches have proven less than satisfactory. In addition, the relatively large number of rolls required for a typical application presents a considerable initial expense and an ongoing maintenance expense, such as, for example, for servicing the large number of bearings associated with the rolls.